1. Field
Certain aspects of the present disclosure generally relate to wireless communications and, more particularly, to methods and apparatuses for supporting adaptive channel state information feedback rate in multi-user communication systems.
2. Background
In order to address the issue of increasing bandwidth requirements that are demanded for wireless communication systems, different schemes are being developed to allow multiple user terminals to communicate with a single access point (AP) by sharing the channel resources while achieving high data throughputs. Multiple Input Multiple Output (MIMO) technology represents one such approach that has recently emerged as a popular technique for the next generation communication systems. MIMO technology has been adopted in several emerging wireless communications standards such as the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard. The IEEE 802.11 denotes a set of Wireless Local Area Network (WLAN) air interface standards developed by the IEEE 802.11 committee for short-range communications (e.g., tens of meters to a few hundred meters).
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min {NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
In wireless networks with a single AP and multiple user stations (STAs), concurrent transmissions may occur on multiple channels toward different STAs, both in uplink and downlink directions. Many challenges are present in such systems. For example, the AP may transmit signals using different standards such as the IEEE 802.11n/a/b/g or the IEEE 802.11ac standards. A receiver STA may be able to detect a transmission mode of the signal based on information included in a preamble of transmission packet.
A downlink multi-user MIMO (MU-MIMO) system based on Spatial Division Multiple Access (SDMA) transmission can simultaneously serve a plurality of spatially separated STAs by applying beamforming at the AP's antenna array. Complex transmit precoding weights can be calculated by the AP based on channel state information (CSI) received from each of the supported STAs.
Since a channel between the AP and a STA of the plurality STAs may vary with time due to a mobility of that STA or due to mode stirring caused by objects moving in the STA's environment, the CSI may need to be updated periodically in order for the AP to accurately beamform to that particular STA. A required rate of CSI feedback for each STA may depend on a coherence time of a channel between the AP and that STA. An insufficient feedback rate may adversely impact performance due to inaccurate beamforming. On the other hand, an excessive feedback rate may produce minimal additional benefit, while wasting valuable medium time.
In a scenario consisting of multiple spatially separated users, it is expected that the channel coherence time, and therefore the appropriate CSI feedback rate, varies spatially across the users. In addition, due to various factors, such as changing channel conditions and mobility of a user, the appropriate CSI feedback rate may also vary temporally for each of the users. For example, some STAs (such as high definition television (HDTV) or set-top box) may be stationary, whereas others (such as handheld devices) may be subject to motion. Furthermore, a subset of STAs may be subject to a high Doppler from fluorescent light effects. Finally, multi-paths to some STAs may have more Doppler than others since different scatterers may move at different velocities and affect different subsets of STAs.
Therefore, if a single rate of CSI feedback is utilized for all supported STAs in a wireless system, the system performance may suffer due to inaccurate beamforming for those STAs with insufficient feedback rates, and/or due to excessive feedback overhead for those STAs with unnecessarily high feedback rates.
In conventional schemes, the CSI feedback occurs at a rate consistent with the worst-case user in terms of mobility or temporal channel variation. For an SDMA system consisting of STAs experiencing a range of channel conditions, no single CSI feedback rate is appropriate for all STAs. Catering to the worst-case user will result in an unnecessary waste of channel resources by forcing STAs in relatively static channel conditions to feedback CSI at the same rate as those in a highly dynamic channel.
For example, in the case of Evolution-Data Optimized (EV-DO) Data-rate Control Channel (DRC), the “channel state” information reflects a received pilot signal-to-interference-plus-noise ratio (SINR) and is transmitted by a STA to facilitate rate selection for the next transmission. This information is updated at a fixed rate for all users, presumably at a rate sufficient to track channel variations associated with the worst-case expected mobility situations. This particular rate of channel state feedback may be unnecessarily high for static users. On the other hand, the DRC was designed to provide a minimal overhead. Because the CSI feedback in SDMA system is used to support complex beamforming at the AP, it may not be feasible to compress or streamline this feedback to a degree accomplished in the EV-DO design.
As another example, for the Institute of Electrical and Electronic Engineers (IEEE) 802.11n standard supporting transmit beamforming, the rate at which CSI is transmitted is not specified, and this is considered an implementation issue. In contrast, due to potentially high overhead of CSI feedback for multiple SDMA users in the IEEE 802.11ac standard, and due to potential abuse of such CSI feedback mechanism by rogue STAs, it may be desirable to specify protocols for CSI feedback in the standard specification.